For papermaking purposes, wood chips, or another fiber source, are ground into smaller chips, or mechanically treated, so that the chips may be broken down further and refined preferably into individual fibers. After refining, these individual fibers are used to make paper or paper-related products, such as paper cups, paper plates, toilet paper, paper towels, diapers, and other products that can be absorbent.
Disc refiners are used to break down clumps of fibers into individual fibers. A disc refiner typically utilizes pairs of opposed refiner discs. A refiner disc is a disc-shaped steel or steel-alloy casting, which has an array of generally radially extending bars or upraised ridges formed in its refining face or refining surface. The refiner disc may be formed of one or more continuous annular discs, or may instead be formed of a plurality of refiner disc segments arranged to form a ring or annulus.
One refiner disc is mounted on a rotor for rotation and the other disc is mounted on another mounting surface opposed to the first refiner disc such that both discs face each other and are very close to each other. The other mounting surface may, for example, be a mounting surface that does not move during refiner operation or another rotor, which turns in a direction opposite the first rotor. As wood pulp passes between the opposed refiner discs, relative rotation between the opposed discs desirably refines the pulp.
Many commercial refiner discs are unidirectional, that is, designed to be rotated only in one direction, or to be stationary and oppose a refiner disc that is rotated only in one direction. Each upraised bar of each disc has a leading edge on one side, where cutting or fibrillation of the fibers being refined primarily occurs, and a trailing edge on the other side. As a result, the leading edge of each bar wears much more quickly than the trailing edge. When too much wear occurs, pulp quality and efficiency dramatically decrease until the refiner disc must be replaced.
While it might seem logical to simply reverse rotation when the leading bar edges become worn to take advantage of the relatively less worn trailing edges, the bars are angled for rotation in only one direction. When unidirectional discs are reversed, which inevitably happens, refining costs rise because refining quality and efficiency suffer. Significantly more power is required to refine the pulp to the desired pulp quality, if the desired pulp quality can even be achieved. Moreover, rotating a unidirectional disc the wrong direction in secondary or rejects refining applications reduces throughput and efficiency and can destroy fiber strength.
Bi-directional refiner discs are designed to be rotated in either direction with the desired goal of extending disc life. Because they are designed to be rotated in either direction, adjacent radial fields of angled bars are symmetrical and mirrored about a radial line. During typical use, a bi-directional disc is rotated in one direction, or faces another bi-directional disc rotating in one direction, for a certain period of time until the leading edges of the bars become worn. The direction is then reversed causing the much less worn and previously trailing bar edges to become the leading edges.
FIG. 2 depicts a prior art segment of a bi-directional refiner disc that is made up of 4, 6, 8, 10, or 12 of these segments. The segment has two fields, I and II, that each have upraised bars that extend radially outwardly and which are mirrored about a radial line, ML. The bars of each field are acutely inclined relative to the mirror line, ML, at about the same angle with the bars in one of the fields angled in one direction and the bars in the other of the fields angled in another direction. The grooves between the bars, through which stock being refined flows, are generally straight with some of the grooves split into two generally straight grooves by a shorter bar. Surface and subsurface dams, respectively indicated by the filled and unfilled circles, are located in the grooves to direct stock flow upwardly toward the bar edges to increase the likelihood that fiber in the stock will be ground between bars of the opposing discs.
During operation, stock is introduced radially inwardly of the disc and flows radially outwardly in the gap between the discs. When the grooves of one of the fields of the opposing disc are generally parallel to the grooves in one of the fields, I or II, stock in that region is urged radially outwardly or pumped. Conversely, when the grooves of one of the fields of the opposing disc cross the grooves in one of the fields, I or II, radial flow of stock is opposed or held back. Because the opposing disc has the same groove and bar configuration as the disc it faces, during disc rotation, the fields I and II alternate between pumping and hold back cycles. When a pumping cycle is occurring in field I, a hold back cycle is occurring in field II, and when a hold back cycle is occurring in field I, a pumping cycle is occurring in field II.
While bi-directional refiner discs have enjoyed substantial commercial success, improvements nonetheless remain desirable. The use of only two fields per disc segment means that when a pumping cycle is occurring in a particular field, it occurs for a certain duration of time. During a pumping cycle, stock flows radially outwardly building momentum. Because the grooves are generally straight, momentum greatly builds as the stock reaches the outer radial periphery of the disc because angular acceleration is greatest in this region. When a hold back cycle begins, the radial outward flow of the fiber is drastically disrupted causing a great deal of the momentum of the stock to be absorbed by the refiner. This results in an increasing load, L.sub.1 (FIG. 3), on the refiner that has a particular amplitude that builds over time until it reaches amplitude, P.sub.1. When another pumping cycle begins, the amplitude of the load reaches a peak, PK.sub.1, and then begins to decrease in the manner depicted by L.sub.1 as the stock begins flowing once again in a radially outward direction. These momentum changes impart load swings that are significant due to the rather large magnitude, P.sub.1 of the load at the time each peak occurs.
These load swings cause vibration that significantly impacts refiner operation. First, the refiner operates less efficiently than desired. Second, pulp quality can undesirably vary. Third, wear is accelerated on the components of the refiner, as well as the refiner disc itself.
In the bi-directional refiner disc shown in FIG. 2, some of the bars extend to the inner peripheral edge of the disc and other bars extend adjacent the edge. Unfortunately, this can impede outward flow of the stock, which can reduce refiner throughput.
To help force the fiber in the stock up into the gap so it gets refined, the refiner disc has over ten rows of dams. Unfortunately, too many dams can obstruct steam flow through the disc. Not only can obstructed steam impede the outward flow of the stock, it can also backflow steam into the stock being fed into the refiner thereby reducing the infeed rate. Moreover, the vibration in combination with obstructed steam can lead to variations in the refining gap, which can further reduce the consistency of pulp quality.